1. Field of the Invention
The present invention relates to a thermal treating apparatus, and more particularly, to a thermal treating apparatus for a semiconductor wafer.
2. Description of the Related Art
In thermal treating steps such as an oxidizing step, a diffusion step, a CVD (chemical vapor deposition) step for an object to be treated, e.g., a semiconductor wafer, a thermal treating apparatus comprising a reaction tube is used. The thermal treating apparatus is operated in the following manner. That is, a boat on which a plurality of semiconductor wafers are stacked is inserted in an opening end of a reaction tube made of quartz glass and wound by a heating wire, the boat is set at a predetermined position in the reaction tube, and the opening end is sealed by a cover plate through a manifold. In this case, the manifold is a ring-like member having a plurality of nozzle ports for supplying plural types of gases in the reaction tube. While the wafer is annealed by heat generated from the heating wire, a predetermined reaction gas is supplied to treat described in Published Examined Japanese Patent the wafers. This thermal treatment technique is Application No. 60-4148 and the Published Unexamined Japanese Patent Application No. 60-119716.
In the thermal treatment technique, in order to prevent the wafer from contamination by a heavy metal, the reaction tube is made of quartz glass. In an etching apparatus disclosed in Published Examined Japanese Patent Application Nos. 57-52423 and 63-41986, an etching chamber is made of SiO.sub.2, Si.sub.3 N.sub.4 or Al.sub.2 O.sub.3.
In a thermal treatment such as CVD, since a reactive gas toxic to human is used, leakage of the reactive gas from the reaction tube must be prevented. For this reason, the opening end of the reaction tube is sealed by a manifold and a cover plate made of stainless steel having an excellent machining precision to obtain air-tightness of the reaction tube.
In the reaction tube, an opening end of which is sealed by the manifold and cover plate made of stainless steel, however, when a thermal treatment such as CVD is performed at a high temperature of 400.degree. to 1,400.degree. C., heavy metals such as Fe (iron) are scattered from the manifold and the cove plate and attached to the wafer surface to contaminate the wafer.
When a silicon layer is to be formed on the wafer surface, an SiH.sub.2 Cl.sub.2 (diclorosilane) gas used as a raw gas is supplied in the reaction tube and dissolved by a plasma to produce Si and HCl, thereby depositing this Si on the wafer. At this time, the manifold and the cover plate made of stainless steel are oxidized by this HCl. In this oxidization, since particles are generated in the reaction tube, the inside of the reaction tube and the wafer are contaminated.